Mars Core

Mars Core is the seventh level of Doom Eternal. After learning from Samuel Hayden that Deag Grav, the final Hell priest, is on Sentinel Prime, the Doom Slayer must proceed to Mars to locate a trans-dimensional gateway to Sentinel Prime, hidden in the planet's core within the lost Night Sentinel city of Hebeth. As VEGA confirms there are no pathways leading there, the Slayer travels to Phobos to gain access to its BFG-10000 orbital defense system and blast a hole in Mars directly to the core.

The BFG-9000 weapon is found in this level. The Maykr drone enemy is also introduced in this level via the Slayer Gate.

"With the help of Samuel Hayden, the final Hell priest has been located on Sentinel Prime - a city from your past. The only known path to this now accursed place is through the dimensional gateway hidden in the core of Mars, wherein the lost city of Hebeth resides. The journey there will take time you can't afford to waste - a dilemma which calls for a more...direct approach."

- Loading description

Lore
Originally established as a mining and communication relay, the UAC outpost on Phobos was expanded following the destruction of the Argent facility on Mars at the request of the UAC's board of directors, who wished to retain their monopoly over the planet. The expansion plan included the addition of a BFG-10000 particle cannon; derived from BFG-9000 technology, this would become the largest man-made weapon platform ever conceived.

The city of Hebeth once bridged a number of disparate Sentinel cities thanks to its slipgate, but was lost in the crusades of the ancient Sentinels. Its ruins have since been buried deep beneath Mars' quaking surface, and nearly all knowledge of it has been lost to time.

Objectives

 * Shoot a hole in Mars
 * Get to the BFG-10000
 * Retrieve the power source


 * Evacuate the Phobos base
 * Reach the portal
 * Use the ion catapult
 * Get to the escape pod

Challenges

 * Big Ba-da Boom - Kill 40 demons with a very big gun
 * Disarmament - Destroy three arm cannon weak points on mancubi
 * Lock and Key - Find the Slayer Key

Master Level
A Master Level version of this mission was released with Update 6.66 on, which makes a number of changes to significantly increase the level's difficulty:
 * The number and variety of monsters is greatly increased, including the whiplash spectre from The Ancient Gods, Part One. Monsters that normally appear later in the campaign are included, including tyrants and arch-viles.
 * Collectibles, challenges, secret encounters and the Demonic Corruption meter are removed, along with the automap station and the Slayer Key. Some items are replaced with soulspheres or spirit armor.
 * Environmental hazards such as laser grids, heavy fog and purple goo are added throughout the level.
 * Ten challenges are introduced which require the player to complete the level at different difficulties, including Ultra-Nightmare. The "Classic Mode" introduced with the Super Gore Nest is retained here, where the player starts with an unmodified combat shotgun (plus all auxiliary weapons and the plasma rifle collected in the opening cutscene) and must collect more weapons and modifications as they progress. Each individual challenge has its own cosmetic item unlock, and completing all ten challenges unlocks a red chrome skin for the plasma rifle.

Pre-release
A shortened version of the first half of Mars Core was the second level of Doom Eternal to be displayed to the public in the BE3 reveal on.

The noticeable differences from the reveal to the final product includes the following:
 * The player takes control of the Slayer before walking inside the elevator, as opposed to starting inside the elevator in the final version.
 * The PA system inside the Phobos outpost has a different voice actor, as do the various UAC scientists and guards in the opening cutscene.
 * The exclusion of the green cracks before exiting out of a vent during the reveal.
 * The exclusion of the UAC Spokesperson hologram before exiting outside.
 * The exclusion of an extra life outside.
 * The exclusion of Samuel Hayden and VEGA throughout the level, presumably to avoid spoilers.
 * The exclusion of the Doom hunter and dread knights near the BFG-10000.
 * The exclusion of the chaingun throughout the level, likely being developed at that time. The final version introduces the chaingun as a mandatory pickup in Super Gore Nest to advance the level, although the part of the level itself was not seen at that time, further supporting the theory.
 * The main entrance to the BFG-10000 is accessible in the reveal, which contains an arch-vile. The Slayer pulls out and swings his Crucible blade in a cinematic, after which the reveal ended. In the final version, however, the main entrance is inaccessible and a maintenance hatch underneath is taken instead. Both the arch-vile and the Crucible are introduced later, on Taras Nabad.

Trivia

 * Development resources and developer-only portions of the level refer to the final Martian core area of the level as "Hell," and indeed, various infernal influences seem to be present in that location in the form of torrents of lava and other small objects which do not obey normal gravity, and a whispering voice that can be heard on the wind when the Slayer first lands his escape pod. The Fast Travel system also refers to this area as Hell, indicating that the nature of the curse on the area is demonic in origin and has lingered on since the fall of the city. The aforementioned whispering heard sounds similar to the voice that narrated the Slayer's Testaments in Doom (2016).
 * Two of the Fast Travel destinations in the meteor section of the level are titled "Alpha Labs" and "Delta Labs". They may be a reference to the Doom 3 locations of the same names.
 * In the Master Level version of this stage, two tyrants appear by the final slipgate. This may have been inspired by a reveal video at E3 2019, which ends with a tyrant emerging from that portal.